Cloudiness is defined based on the proportion of the sky covered with clouds. It is visually determined by observers at meteorological stations. Observation of the sky and estimation of cloud coverage results in cloudiness data meteorologically expressed in tenths. Clear day is a day with cloudiness less than two tenths (2/10), while cloudy is a day with cloudiness above 8 tenths (8/10).
There are two basic types of cloudiness: static (inversion), characterised by persistent layered clouds covering the entire sky, and dynamic, characterised by puffy clouds that cover only portions of the sky.
Cloudiness affects duration of insolation, daily exposure to light, atmospheric counter-radiation, etc. (Penzar, 2000). It is important for many human activities - tourism, health, energy, road traffic, and especially air traffic.
Clouds and cloudiness are particularly important in the relationship between ground surface radiation and atmospheric radiation. They reduce solar radiation to Earth, but also prevent long-wave radiation to higher layers of the atmosphere. Increased cloudiness brings air temperature into a narrower range which means tampering of both positive and negative extremes. Therefore, cloudiness during the day reduces warming up of the air and earth surface, and during the night, it reduces cooling.
According to the distribution of the mean annual cloudiness, the territory of the Federation of Bosnia and Herzegovina can be divided into a clearer and a cloudier part. The clearer part is an area where mean annual cloudiness in, on average, less than five-tenths (5/10) and covers borderline areas of karts fields in the southwest, west Herzegovina, area around Mostar, and the Neretva valley all the way to Neum. Minimum cloudiness is typical of Neum, with mean annual value of 3 to 4 tenths (3-4/10). The rest of the territory of the Federation of Bosnia and Herzegovina, with mean annual cloudiness above 5 tenths (5/10), is considered the cloudier part.
According to the annual distribution of cloudiness, December is the cloudiest month. As March approaches, cloudiness gradually decreases, however, in April, May and July, it increases again. Spring increase in cloudiness is caused by cyclones transiting above this area. Summers are clearer due to the Azores High moving north which includes the territory of Bosnia and Herzegovina. This reduces transition of barometric depressions to a minimum. This particularly applies to southern areas. As we go north, the relief changes and local modifiers become increasingly important in terms of causing cloudiness, especially in the central parts of the country.
From October onwards, cloudiness increases significantly due to Mediterranean circulation of air. Autumn is somewhat less cloudy compared to spring.
In winter, due to low inversion cloudiness, a major part of the sky is cloudier in lowlands than in highlands. Winter months on high mountains record somewhat lower levels of cloudiness compared to spring months. Winter is the cloudiest, not only due to barometric depressions, but also due to anticyclones causing a lack of precipitation but forming low stratus clouds and fogs.
As the latitude increases, mean monthly cloudiness on the territory of the Federation of BiH also increases from 2 tenths (2/10) in summer period in the south, to 8 tenths (8/10) in the north, during winter months. Maximum mean annual cloudiness of 7 tenths (7/10) is recorded in Gračanica, while the annual minimum of 4 tenths (4/10) is recorded in Domanovići, Neum, Ljubuški and Posušje; however, mean monthly cloudiness in Domanovići, all year round, is less than 5 tenths (5/10). Deeper in the continent, Gračanica, Donji Vakuf, Jajce and Vitez do not have a month a year with cloudiness less than 5/10. Southern edges of the Dinaric system, the area of the Nišići plateau and the Tuzla basin, in late spring, during summer and in the major part of autumn, record low values of cloudiness, ranging between 3 (3/10) and 5 tenths (5/10). The majority of the FBiH territory records average annual cloudiness of 5 to 6 tenths.
Distribution of clear days is usually opposite of distribution of cloudy days, and, for the most part, it matches the general atmospheric features of our region. In December, the number of cloudy days peaks, and then gradually decreases towards summer, while the situation is quite the opposite with clear days. At the majority of stations in the Federation of BiH, in the sunniest summer months of July and August, the average number of clear days is greater than the number of cloudy days. Besides Bjelašnica, exceptions include stations in Gračanica, Fojnica and Jajce, where these two months record more cloudy days than clear days.
Greater number of clear days compared to cloudy days, is recorded also in September, at some stations deep in the continent (Livno, Makljen, Tuzla, Travnik, Sarajevo). Going from the north southwards, the average number of clear days, compared to cloudy days, increases even sooner, with spring months. Prozor, Tomislavgrad, Mostar, Stolac - stations that register more clear days than cloudy days a month, from June to October.
The longest period of 6 months (May – October), in which the average shows more clear than cloudy days a month, is recorded at the Čapljina station in the Neretva valley. Compared with Mostar, the period of prevailing clear weather is longer by a month, which is evident from the graph below. The reason for this can be found in the openness of Čapljina towards the sea, where streams of hot air prevent formation of vertically-developed clouds above the town.
The average annual number of clear days in the Federation of Bosnia and Herzegovina ranges from 140 in široki Brijeg to 29 in Gračanica. Extreme conditions in the distribution of clear and cloudy days are registered in the Neretva valley, west Herzegovina, and in the narrow coastal area in the south of the country on one side, and parts of the central and NE Bosnia on the other. The average number of clear days in Ljubuški is 138, Čapljina 130, and Posušje 123. Central parts of Bosnia are under great influence of the static conditions caused by many cloudy days due to frequent formation of stratus clouds, high and low fog. In this area, the number of clear days is smaller, ranging between 31 in Jajce, 45 in Zenica and 61 day in Sarajevo. The northeast part of Bosnia also records small annual number of clear days, less than 50 (Bihać 48, Sanski Most 41, Ključ 45).
The average annual number of cloudy days ranges between 66 in Posušje and 160 in Gračanica. Almost entire Bosnia has 120 or more cloudy days a year, with an exception of certain locations in the southeast, where this number is smaller (Drvar 114, Livno 107). In the most of Herzegovina, the number of such days is less than 100 (Mostar 98, Stolac 95, Čapljina 80, Posušje 66).